Have a video sequence be driven by an audio track
The problem is:

The scene isn´t updated because the sequencer ignores the scene when it´s not visible in the final output. We might consider this a bug/limitation or a feature. I´ll talk to the core developers. So of course drivers that depend on the values of a hidden scene don´t work as expected.

A workaround: 

If you don´t use the scene for output, render the scene so that the output is pure white and add the scene in the sequencer on top of every channel and use multiply as blend mode. Of course this increases the render time a bit. The key point here is that the scene shows up one way or another in the final render.